Large Scale Ground Deformation of Etna Observed by GPS Between 1994 and 2001

Nicolas Houlié, Pierre Briole, Alessandro Bonforte and Giuseppe Puglisi

Introduction

Etna is the most active volcano in Europe. Several active tectonic structures are located in its eastern part. Some of these structures, such as the Timpe fault system (extensive fault system trending NNW-SSE belonging to the Maltese escarpment) and the NNE-SSW faults (belonging to the Messina-Comiso line) were inherited from its geodynamic setting (Monaco et al., (1997), Laigle, 1998, Nicolich et al., 2000, Jacques et al., 2001). Others, such as the Valle del Bove (Calvari et al., 1998), the Pernicana fault system (Azzaro et al., 1998, 2001a, 2001b), and the rift zones (Tibaldi and Gropelli, 2002), are linked to Mt. Etna's activity.

The Etna volcano GPS network, conceived in the late eighties, improved and maintained by Istituto Nazionale di Geofisica e Vulcanologia (INGV) research team, is composed of two main parts. Firstly, a local reference frame, relatively far from Mt. Etna's influence and assumed stable. Secondly, a monitoring network on the volcano dedicated to the study of the volcano dynamics (Puglisi et al., 2004). Thus, this network is able to detect both volcanic and tectonic deformations of the area.

The last three flank eruptions of Mt. Etna occured in 1991-1993, in July 2001 and november 2002 (Branca et al., 2004). The time interval of our study was chosen to investigate the deep magma plumbing system of Etna (Patane et al., 2000) while the GPS network remained stable(Puglisi et al., 2004).

Data and data processing

Between 1994 and early 2001, thirty GPS campaigns were carried out by the INGV research group at twenty-three different benchmarks. All INGV receivers were Trimble 4000 SST/SSI. The various campaigns used in this study were designed for various aims (i.e. monitoring surveys, specific experiment in support to kinematic surveys or photogrammetry). Therefore, they were not homogenous in terms of duration of observations, sampling rates, number of measured benchmarks, number of instruments involved. Typically measurement sessions last two to four hours at 10, 20, or 30 seconds sampling rate. Station 98, located on the roof of the INGV building in Catania, records continuously during the campaigns.

To tie the local network to the European Reference Frame (EUREF), we have processed the available data from permanent sites located in southern Europe with our local dataset using GAMIT software (King and Bock, 1999). All the ambiguities have been fixed for baselines shorter than 500km only. Adjusting the computed baselines for each campaign using the GLOBK software (Herring, 2005), we established a set of coordinates for each campaign for the points measured on Etna as well as for the International GPS Service (IGS) stations.

Figure 19.1: Velocities computed from time-series (reference NOTO). The location of the source is symbolized by a black star.
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The velocities are radially distributed (Figure 1) and seem to be organized as the result of a ponctual source in a overpressure state located beneath Mt. Etna. As the state of this deep source was already discussed by Patane et al., (2000), we have chosen to model the velocity field associated to the over-pressure of a Mogi point source (Mogi, 1958).

The best-fit solution of the Mogi point is located near the summit of the volcano (East 499.0 km, North 4180.5 km UTM33) located $9.5\pm1 km$ beneath the summit assuming a vertical maximal velocity of $80\pm5 mm/y$ (Figure 2). The depth of this source is in agreement with the results of several studies carried out by modeling ground deformation data (both GPS and INSAR) (Bonaccorso et al., 1996, Lanari et al., 1998, Puglisi et al., 2001, Bonforte et al., 2003, Lundgren et al., 2003, Lundgren et al., 2004). The vertical accuracy of the GPS velocities were not accurate enough to test the impact of the topography of the volcano on our modelling. However, the numerical simulations of the impact of the topography on the deformation field allow us to estimate that the computed vertical maximal inflation were overestimated of 30 percent near the summit ($24 mm/yr$).

The point source model explains the observations except in the eastern part of the volcano (Sites 15 and 22) and along the Pernicana fault system (Site 27, Figure 19.2). The fact that the model doesn't fit exactly in the eastern part of the network along the Ionian coast is in agreement with the eastward movement of the eastern part of the volcano toward the sea (Rasa et al., 1996, Froger et al., 2001, Puglisi et al., 2003) . The magnitude of the site 15 's velocity ($11 mm/y$ to the East) supports the hypothesis of the existence of a large slough located along the Ionian coast limited by the Pernicana fault to the north, Ionian coast to the East and Timpe fault system to the West. The volume of this slough was estimated from $5$ to $50km^3$ (Houlié, 2005) while the mechanism driving the dynamic of these units is not clearly identified yet.

Figure 19.2: Data (black filled circles) and best model (black lines) plotted versus the horizontal distance from the source (Mogi, 1958). The computed horizontal displacements are symbolized by a continuous black line while the vertical one by a dashed one. Three sites dynamics don't fit with the modeled displacements (15,22 and 27). Those are highlighted by grey filled black circles.
[width=0.8]houlie_et_al_figure5_sub.eps

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